Image suppression system



March 2, 1943. D. v. SINNINGER IMAGE SUPPRESSION SYSTEM Filed Oct. 16, 1940 INVENTOR. LW/GHT l/ 5 M N MW Patented Mar. 2, 1943 IMAGE SUPPRESSION SYSTEM Dwight -V. Sinninger, Chicago, 111., assignor to Johnson Laboratories; Inc., Chicago, 111., a corporation of Illinois Application October 16, 1940,'Serial No. 361,352

10 Claims. 01. 250-20) The present invention relates particularly to receiving systems of the superheterodyne type in which the signal passes through one Or more resonant circuits that are tuned to the signal frequency and is mixed'wlth locally generated oscil- 1 lations to produce a predetermined beat frequencyat which it is amplified and which is generally termed the intermediate frequency. The part of the system through which the signal passes before any change in frequency occurs usually includes an antenna circuit and one or more resonant circuits with or without thermionic amplifying vacuum tubes and is generally termed the preselector.

When a superheterodyne receiver is tuned to a desired signal some response may be obtained from a signal which differs from the frequency of the desired signal by twice the intermediate frequency of the receiver. In the usual arrangement the local oscillator is adjusted to a frequency above, rather than below, that of. the signal to be received. If such a system is tuned, for example, to receive a signal having a frequency of 1000 kilocycles, and if the intermediate frequency of the receiver is 460 kilocycles, the local oscillator will be adjusted to a frequency of 1460 kilocycles in order to convert the received signal to the intermediate frequency of 460 kilocycles,,-at which it is to be further amplified. If, at the same time, a signal of 1920 kilocycles happens to be present at the input of the receiver, this signal, together with the oscillations of the local oscillator, will likewise produce a beat frequency of 460 kilocycles which will also be amplified by the intermediate frequency portion of the receiver, and will supply an undesirable response at the output thereof. This type of interference is commonly referred to as image response.

In order to eliminate such interference, the preselector of the receiver may be provided with a so-called rejector circuit or image frequency trap that is tuned to the image frequencyand is so disposed as to block the path from the antenna to the grid of the first vacuum tube for signals of the image frequency. Since the preselector is tunable over a range of frequencies, the image rejector circuit must likewise be tunable over a different, but properly related, range of frequencies, if it is to be fully effective.

The frequency range that the rejector circuit has to cover is usually higher and narrower than the range over which the preselector operates. For example, if a receiver having an intermediate frequency of 460 kilocycles is tunable over a range of from 540 kilocycles to 1650-ki1ocycles,

' receiver.

the corresponding image frequency range extends from 14.60 kilocycles (='540+920) to 2570'kilocycles (=1650+920), and'consequently the'frequency ratio of the rejector circuit is only 1.76 as against a ratio of 3.06 for the preselector circuit. Hence in the particular example under discussion, either the inductance or the capacitance of the preselector circuit must be increased about 9 times to tune over the signal frequency range, whereas the inductance or capacitance of the rejector circuit requires to be increased only about three times to tune the circuit over the corresponding'image frequency range. For proper image frequency suppression, it is necessary that the inductance or capacitance changes in the preselector and rejector circuits occur in such amounts, and at such rates,

respectively, that for any signal frequency to which the preselector circuit may be tuned, the image rejection circuit is resonant ata higher frequency which differs from the signal frequency by twice the intermediate frequency of the This requirement for substantially exact alignment of the preselector and rejector circuits at afixed. frequency'difference throughout the-range of adjustability is not readily met with simple"andiinexpensive means, and constitutes a major problem in the design of such systems, if both circuits are to be simultaneously tuned by unitary control means.

It is an object of the present invention to provide an arrangement for superheterodyne receivers and the like by which two circuits may be simultaneously tuned over different but properly related frequency ranges by a single reactancevarying element.

It is another object of the present invention to provide a uni-control preselector-rejector system for superheterodyne receivers which aifords effective suppression of image frequency response over the entire tuning range of the receiver.

In accordance with the invention, I provide a system including a preselector circuit and an image rejector circuit wherein both circuits are tuned simultaneously by movement of a common ferromagnetic core relatively tov their inductance l coils, the arrangement being such for any setting of the core, that the two circuits are tuned to frequencies that differ from one another by the required constant frequency interval.

The system in-accordance with the present invention icomprises a signal reception circuit having an inductance coil connected between the signal collecting means and the grid of the first *vacuum tube, a separate parallel resonant circuit the core upon said winding will be materially reduced and hence the range of inductance variations obtainable, in the absorption circuit by movement of the core will be materially decreased, whereas the signal reception circuit has practically the full benefit of the effective permeability of the core as to its tuning range. Thus, by choosing the appropriate capacitances to be connected with said coil and said'winding respectively,.the signal receptioncircuit and the image absorption circuit may be made to resonate atthe highest signal frequency and the corresponding image frequency respectively of a receiver with the core in its initial position, and by properly proportioning and positioning the coil, the winding and the core in themselves and relative to one another, the two circuits may be simultaneously tuned by movement of said common core over the signal and image frequency ranges respectively while substantially maintaining the required frequency difference between them. In order to obtain such inductance changes in the absorption circuit with movement of the common tuning core, as will guarantee perfect alignment of the two circuits at any frequency within the tuning range, the winding of the absorption circuit may be given a varying winding pitch with more turns per unit length at the end at which the core enters first than at any other end. Alternatively, the winding of the absorption circuit may be given greater length axially than the coil of the reception circuit, or

may be displaced axially relatively to said coil so as to project beyond the same at one end thereof with the core so disposed as to extend inside the projecting portion of said winding in its initial position. Any of these expedients may be employed separately or all of them may be employed together with a view of maintaining a substantially constant frequency interval between the two circuits throughout the tuning range.

The invention will best be understood by reference to the accompanying drawing which illustrates preferred embodiments thereof and in which Fig. 1 is a schematic diagram of a preselector system tuned by the relative movement of a coil and a ferromagnetic core, and including an arrangement for the rejection of the image frequency in accordance with the invention;

Fig. 2 is a modification of the arrangement of Fig. 1, and

Fig. 3 is a longitudinal section of a variable inductance device such as may be employed in the arrangement of Fig. 2.

In prior known systems, an inductance coil and two capacitors formed a resonant circuit which included the signal collecting means, and was tuned to the signal to be received by movement of a ferromagnetic'core relatively to the coil. At the same time the coil and one of the capacitors formed a parallel circuit which was intended to resonate at the image frequency and to act as an image rejector circuit. It is apparent that in '75 .example, if the effective permeability of the core was .9, it would tune the preselector circuit, say from 1650 kilocycles to 540 kilocycles and a frequency change of the same ratio would occur in the image rejector circuit, so if the image rejector circuitresonated initially to a frequency of 2570 kilocycles, with the core fully inserted it would resonate at a frequency of 857 kilocycles, although the image rejector circuit should have been tuned over a range of from 2570 kilocycles to 1460 kilocycles only. The required interval of twice the intermediate frequency would exist, therefore, only at the upper end of the frequency range and as the frequency of the preselector circuit was decreased, the deviation of the rejector circuit from its required frequency would become increasingly pronounced, with the result that its capability of suppressing image frequency signals would be greatly decreased. The present invention entirely overcomes this important defect of earlier systems otherwise superficially similar.

Reference is now made to Fig. 1 in which inductance coil I, capacitor 2 and signal collecting means 3 are connected to form a resonant circuit, control grid 4 and cathode 5 of vacuum tube 6 being connected across capacitor 2 in the conventional manner. Cathode resistor I is bypassed by condenser 8, and resistor 9 provides a direct current path from a source of bias voltage (not shown) to control grid 4. Signal collecting means 3 may be an exposed inductive element commonly called a loop, or it may be a capacitive antenna.

Associated with inductance coil I there is a winding I0 coaxial with coil I and of larger diameter. A capacitor II is connected across winding ID to form a parallel circuit. Due to the close proximity of coil I and winding I9, parallel circuit I0, II operates to absorb from circuit I, 2, 3, signals of any frequency to which said circuit I0, II may be tuned. A ferro-magnetic core I2 is movable relatively to coil I and Winding I0 to simultaneously vary their inductances.

By proper choice of the inductances of inductors I and I0 with the core withdrawn, and of the capacitances of capacitors 2 and II, circuit I, 2, 3 and circuit I0, II may be made to resonate at a signal frequency and at the corresponding image frequency respectively at the upper end of the tuning range of a receiver, and by proper choice of the diameter of winding III relative to core I2 the inductance increasing effect of core I2 in respect of winding I0 may be so curtailed that when core I2 is fully inserted the image circuit will very accurately resonate at the image frequency corresponding with the signal frequency to which preselector circuit I, 2, 3 has been tuned by insertion of the same core I2 into coil I.

' However, the problem in providing proper image suppression over the total tuning range of a cuit that it will be tuned over a narrower frequency range by the same control by which the signal frequency circuit is tuned, but additionally in so arranging matters that as the absorption circuit is tuned it is maintained in continuous alignment at the required frequency'difierence with respect to the signal frequency circuit. In other words, although proper alignment may have been established at both ends of the tuning range by proper curtailment of the range covered by the image trap circuit, the required frequency difference will not be maintained at intermediate points of the range unless other and additional expedients are employed. In general in order to secure continuous alignment it is necessary to increase the rate of inductance change in the image rejection circuit in the high frequency region of the tuning range and to decrease the rate of inductance change in said circuit in the low frequency region of the tuning range.

In accordance, with the invention the necessary correction of the rate of change of inductance produced in winding ID by movement of core l2 to yield continuous alignment at a fixed frequency difference with respect to signal circuit l, 2, 3 throughout the tuning range of a receiver, is secured by giving winding ID a varying pitch, 1. e. winding may be given more turns per unit length at the end at which core 12 enters and less turns per unit length at the remote end, as schematically shown in Fig. 1. Thus, by the simple expedient of properly choosing the diameterand suitably varying the pitch of winding 10 I am able to tune absorption circuit I0, I I over the required image frequency range in continuous alignment at a fixed frequency interval as against signal circuit I, 2, 3.

The modification indicated diagrammatically in Fig. 2 illustrates an arrangement wherein absorption circuit l3, [6 may be imparted suitable alignment characteristics without the use of a varying pitch for winding 13. In the arrangement there shown winding I3 is made somewhat longer than coil M and is placed in such a position with respect to coil l4 that when core I5 is fully withdrawn from coil 14 at the high frequency end of the range it is nevertheless partly within winding 13. l'his not only decreases the frequency range over which absorption circuit is, H5 will be tuned by motion of core 15, but by properly dimensioning and positioning winding 13 relative to coil l4 and core l5, it permits of bringing about such an alteration of the rate of the inductance change produced in winding 13 by motion of said core 15 compared with that effected in coil 14 by motion of the same core, as to produce satisfactory alignment of the two circuits at the required fixed frequency difference throughout the tuning range of the receiver.

In Fig. 2 the signal collecting means is shown as a capacitive antenna ll having blocking capacitor I8 and coupling capacitor I9.

Absorption circuits l0, H and I3, 16 respectively may suitably be grounded as shown in the figures.

In certain cases, the capacitance of the absorption circuit may be only the distributed capacitance of the Winding, thus saving the cost of capacitors H and IS in Figs. 1 and 2 respectively. It will usually be found desirable, however, to provide a small adjustable capacitor to facilitate initial establishment of the required frequency difference.

In Fig. 3 there is shown a coil winding and core combination suitable for use in the arrangement described in connection with Fig. 2, the arrangement of the parts being indicated by the reference numerals. The constructional data for a device such as shown in Fig. 2 that is intended for a signal frequency range of from 540 to 1650 kilocycles and an intermediate frequency of 460 kilocycles, may suitably be as follows:

Coil 14 Outside diameter of tube, .221 inch Length of winding. 1.125 inches Winding, progressive universal Turns, 357 of 7-41 single silk enameled Litz wire Inductance, 152 microhenries Winding 13 Outside diameter of tube, .340 inch Length of winding, 1.4375 inches Winding, close wound single layer Turns, 164 of No. 36 plain enameled wire Displacement from coil l4, D=.1875 inch Core 15 Diameter, 0.200 inch Length, 1.250 inches Capacitor I8, 200 micromicrofarads Capacitor I9, 500 micromicrofarads Resistor 20, 25,000 ohms Capacitors 2, H, l 6 and 21 may suitably be conventional two-plate trimmer condensers.

It will be understood that while I have given specific data for a device suitable for use in one embodiment of the invention, I do not limit myself to this construction, since numerous other modifications and variations will suggest themselves to those skilled in the art.

It will also be understood that the system of the present invention is not limited in its uses to the preselector stage of a superheterodyne receiver. It may be employed wherever there is a tendency in systems of the wireless reception art for interference to occur on frequencies that are a substantially equal frequency interval apart from the desired signals.

Having thus described my invention what I claim is:

1. A circuit arrangement for the suppression of undesired responses in radio receivers and the like including a signal collecting means, a vacuum tube, a resonant circuit having an inductance coil, a conductively' separate parallel resonant.

absorption circuit having a winding of a larger diameter than, and surrounding said inductance coil, and a ferromagnetic core arranged to be movable relatively to said coil and said winding, the diameters of said coil and said winding being so proportioned relative to one another that movement of said core tunes said first resonant circuit over a range of desired frequencies and simultaneously tunes said second resonant circuit over a range of undesired frequencies.

2. A circuit arrangement for image frequency suppression in superheterodyne receivers and the like including a signal collecting means, a vacuum tube, a resonant circuit having an inductance coil connected between said signal collecting means and the grid of said tube, a conductively separate parallel resonant circuit having a winding of larger diameter than, and surrounding said inductance coil, and a ferromagnetic core movable relatively to said coil and said winding, the diameters of said coil and said winding being so proportioned relative to one another that movement of said core tunes said first resonant circuit over a range of signal frequencies and simultaneously tunes said second resonant circuit over the corresponding range or" image frequencies.

3. A circuit arrangement for radio receivers of undesired responses in radio receivers and the like including a resonant circuit having an inductance coil, a separate parallel resonant circuit having a winding surrounding said coil and being of varying winding pitch, and a ferromagnetic core arranged to be movable relatively to said coil and said winding in order to simultaneously tune said circuits, said winding having more turns per unit length at the end into which said core enters first than at the other end.

5. A circuit arrangement for image frequency suppression in superheterodyne receivers and the like including a signal collecting means, a vacuum tube. a resonant circuit having an inductance coil connected between said signal collecting means and the grid of said vacuum tube, a separate parallel resonant circuit having a winding surrounding said inductance coil and being of a varying winding pitch, and a ferromagnetic core arranged to be movable relatively to said coil and said winding in order to tune said first resonant circuit over a range of signal frequencies and to simultaneously tune said second resonant circuit over the corresponding range of image frequencies in perfect alignment throughout the tuning range.

6. A circuit arrangement for image frequency suppression in superheterodyne receivers and the like including a signal collecting means, a vacuum tube, a resonant circuit having an inductance coil connected between said signal collecting means and the grid of said vacuum tube, a separate parallel resonant circuit having a winding surrounding said coil, and a ferromagnetic core arranged to be movable relatively to said coil and said winding in order to tune said first circuit over a range of signal frequencies and to simultaneously tune said second circuit over a corresponding range of imagefrequencies, said winding having more turns per unit length at the end at which said core enters first than at the other end.

'7. A circuit arrangement for the suppression of undesired responses in radio receivers and the like including a signal reception circuit having said inductance coil, said winding beinglongeraxially than said coil so as to project beyond the same at one end thereof, and a ferromagnetic core disposed to extend inside the projecting portion of said winding and arranged to be 'movable into said coil so as to tune said circuits over different frequency ranges while maintaining a substantially constant frequency interval between said circuits throughout the tuning range.

8. A circuit arrangement for the suppression of undesired responses in radio receivers and the like includin a signal reception circuit having an inductance coil, a conductively separate parallel resonant circuit having a winding arranged around and in axially displaced relation relative- 1;? to said coil so as to project beyond the same at one end thereof, and a ferromagnetic core disposed to extend inside the projecting end of said winding and arranged to be movable into said coil to tune said circuits over different frequency ranges while maintaining a substantially constant frequency interval between said circuits throughout the tuning range.

9. A circuit arrangement for the suppression of undesired responses in radio receivers and the like including a signal reception circuit having an inductance coil, a conductively separat parallel resonant circuit including a winding of larger diameter and greater length than said coil, said winding being placed around said coil in axially displaced relation thereto so as to project beyond the same at one end only, and a ferromagnetic core disposed to extend inside the projecting portion of said winding and arranged to be movable into said coil so as to tune said signal reception circuit over a range of lower frequencies and to simultaneously tune said separate parallel resonant circuit over a range of higher frequencies with a substantially constant frequency interval between said circuits throughout the tuning range.

10. A circuit arrangement for image frequency suppression in superheterodyne receivers and the like including a signal collecting means, a, vacuum tube, a resonant circuit having a coil connected between said signal collecting means and the grid of said vacuum tube, a conductively separate parallel resonant absorption circuit including a winding of larger diameter and greater length than said coil and placed around said coil in axially displaced relation thereto so as to project beyond one of the coil ends only, and a ferromagnetic core disposed to extend inside the projecting portion of said Winding and arranged to be movable into said coil so as to tune said first resonant circuit over a range of signal frequencies and to simultaneously tune said second resonant circuit over the corresponding range of image frequencies.

DWIGHT V. SINNINGER. 

